1. Field of the Invention
The present invention relates to a transmission line, and particularly, to a transmission line having a photonic band gap (PBG) coplanar waveguide (CPW) structure and a method for fabricating a power divider using the same.
2. Description of the Prior Art
In general, most important factors in designing a transmission line are a correct characteristic value and a maximum received power of a transmission line. The maximum received power means a limit of power a transmission line can endure. That is, even though a transmission line has an accurate characteristic value, if power higher than the maximum received power is applied to such a transmission line, the transmission line itself is broken.
FIG. 1 illustrates a transmission line having a general coplanar waveguide (CPW),
As shown therein, a signal applied to an input terminal 11 of a transmission line 10 generates an electric field and a magnetic field through an interval 14 between a signal line 13 and a ground-conductive layer 16 to be transmitted to an output terminal 12 of the transmission line 10. At this time, a characteristic impedance value of the transmission line 10 is determined by a width of the signal line 13, a distance 14 between the signal line 13 and a ground-conductive layer 16, a thickness of a substrate 17 and a dielectric constant of the substrate 17.
In addition, in case that a dielectric constant of the substrate 17 is high, a width of the signal line 13 is remarkably narrowed. That is, if power is applied to the signal line 14 having a small width (e.g., 2.5 μm), the signal line 13 may be short-circuited by heat. For example, on the assumption that a distance between ground-conductive layers 16 is 240 μm, the width of the signal line should be smaller than 2.5 μm in order to implement a CPW having a characteristic impedance of 132 ohms or more on a GaAs substrate having a thickness of 625 μm and a dielectric constant of 12.9. But, it is very difficult to implement a signal line having a width of 2.5 μm or less. Accordingly, a CPW having a signal line a width of which is wide and a high characteristic impedance is needed.
Recently, a CPW structure to which a Photonic Band Gap (PBG) is applied has be proposed. The PBG CPW structure is used as a band-stop filter having a reduction characteristic for a specific frequency band.
The PBG suppresses an advance of an electromagnetic wave and changes an impedance and a phase of a transmitted signal. The PBG structure is mainly applied to an antenna having a microstrip form, a resonator, a filter or the like.
However, the PBG CPW structure according to the conventional art cannot obtain a high characteristic impedance. That is, in the PBG CPW structure in accordance with the conventional art, a width of a signal line should be narrow in order to obtain a high impedance. But if the width of the signal line is narrow, it is difficult to apply the PBG CPW structure to a passive circuit of a micro wave band or a millimeter wave band.
Accordingly, it is difficult to apply a transmission line having the PBG CPW structure or having a general. CPW structure to an unequal Wilkinson power divider requiring a high impedance characteristic. In addition, since the transmission line having the PBG CPW structure or having a general CPW structure cannot have a high characteristic impedance, it is also difficult to apply such a transmission line to an antenna or a filter requiring a high characteristic impedance.
As so far described, in the coplanar wave guide (CPW) in accordance with the present invention, if a characteristic impedance is increased, a width of a signal line should be remarkably reduced. In addition, if a characteristic impedance is increased in a high frequency band, a width of the signal line should be extremely reduced. For this reason it is difficult to implement a transmission line having a high impedance.
In addition, since a power divider in accordance with the conventional art dividea high power in a high frequency band and requires a high characteristic impedance, it is difficult to apply the transmission line having the PBG CPW structure in accordance with the conventional art to the power divider. That is, a signal line having a narrow width can be easily broken and is hard to fabricate.
A transmission line in accordance with another conventional art is disclosed in U.S. Pat. No. 6,518,864 registered on 11st Feb., 2003.